1. Field of the Invention
The present invention relates generally to design and operation of nuclear reactors, and more particularly to a method of determining a margin to an operating limit of a nuclear reactor.
2. Description of the Related Art
During the operation of a boiling water reactor (BWR) or pressurized water reactor (PWR), a continuous monitoring of all operating parameters and resulting thermal limits is performed. For example, percent rated power, percent rated flow, inlet moderator temperature, core pressure, and any positioning of control blades are monitored in order to identify the instantaneous status of the reactor. Also, instrumentation within the reactor core helps monitor reactivity, which maps to corresponding operating responses in parameters such as critical power ratio (CPR), Maximum Average Planar Linear Heat Generation Rate (MAPLHGR), and Maximum Fraction of Linear Power Density (MFLPD), each of which represents a core safety thermal limit on nuclear fuel and which may also be referred to as power-related limits on nuclear fuel. These measured thermal responses are compared to their corresponding operating limits to provide the current margin to the operating limits. The continuous monitoring of core parameters and corresponding margins to operating limits is done throughout the core energy cycle. A computer which performs this monitoring is called a “process computer”. At a minimum, snapshots of the reactor status and resulting margins to operating limits such as the thermal limits above and/or operating limits are processed once per day and stored, typically as an electronic ASCII file.
In order to maintain models of the reactor for use in projection work, development of subsequent design cycles, and/or to provide support for current operating issues, designers or plant operators maintain an off-line (not on the process computer) three dimensional (3-D) simulation of the reactor that resembles the actual operation of the given cycle in the actual reactor core. There are typically differences between the thermal and reactivity margins determined by the process computer (measured margins to operating limits) and those predicted by the off-line model (predicted margins). These differences are caused by a variety of factors, including inadequacies in simulator models, imperfect modeling of the actual plant operation, uncertainties in operating parameters, uncertainties in tip measurements, etc., as well as other unknown uncertainties. Differences between the on-line and off-line margins (i.e., to thermal, reactivity and/or power-related operating limits) determinations force plant operators to require additional margin to these operating limits, so as to insure trouble free operation. Additional margin can be obtained by making changes to the operational parameters, and/or by selection and positioning of different rod patterns. However, the cost of such changes typically is a loss of power or fuel cycle efficiency. Moreover, a “larger than needed” margin requirement has an adverse economic impact on the plant.
The determination of sufficient operating limit margins and predicted trends for expected operating limits and uncertainty exposure dependent biases is a complex problem for design and operation of a nuclear reactor. From the time of the first nuclear reactor it has been observed that the predicted results from computer models and the observed reality (actual operating limits as determined from on-line operation) can oft-times be significantly different for these important dependent variables (i.e., operating limits). To protect against these differences, engineers have developed standard design margins or historical design margins that are to be used to account for or “cover” these differences.
However, these standard design margins are crude at best. Sometimes, the historical required design margin is inadequate, resulting in manipulation of control rods during operation in order to regain lost margin. If rod pattern changes do not alleviate or correct the problem, plants have been even known to have to de-rate (lower power production). Either solution is extremely costly to the fuel cycle efficiency and can cost millions of dollars in lost revenue. Additionally, the historical design margin is occasionally inappropriately conservative, thereby resulting in a reduction in possible fuel cycle efficiency.